STRONG AFTERSHOCKS (Magnitude 5 and larger) -

At this time (immediately after the mainshock) the probability of a
strong and possibly damaging aftershock IN THE NEXT 7 DAYS is
approximately 30 PERCENT.

EARTHQUAKES LARGER THAN THE MAINSHOCK -

Most likely, the recent mainshock will be the largest in the sequence.
However, there is a small chance (APPROXIMATELY 5 TO 10 PERCENT) of an
earthquake equal to or larger than this mainshock in the next 7 days.

WEAK AFTERSHOCKS (Magnitude 3 to 5) -

In addition, approximately 12 to 40 SMALL AFTERSHOCKS are expected in
the same 7-DAY PERIOD and may be felt locally.

This probability report is based on the statistics of aftershocks typical for
California. This is not an exact prediction, but only a rough guide to
expected aftershock activity. This probability report may be revised as more
information becomes available.

Background Information About Aftershocks

Like most earthquakes, the recent earthquake is expected to be followed
by numerous aftershocks. Aftershocks are additional earthquakes that
occur after the mainshock and in the same geographic area. Usually,
aftershocks are smaller than the mainshock, but occasionally an
aftershock may be strong enough to be felt widely throughout the area
and may cause additional damage, particularly to structures already
weakened in the mainshock. As a rule of thumb, aftershocks of
magnitude 5 and larger are considered potentially damaging.

Aftershocks are most common immediately after the mainshock; their
average number per day decreases rapidly as time passes. Aftershocks
are most likely to be felt in the first few days after the mainshock,
but may be felt weeks, months, or even years afterwards. In general,
the larger the mainshock, the longer its aftershocks will be felt.

Aftershocks tend to occur near the mainshock, but the exact geographic
pattern of the aftershocks varies from earthquake to earthquake and is
not predictable. The larger the mainshock, the larger the area of
aftershocks. While there is no "hard" cutoff distance beyond which an
earthquake is totally incapable of triggering an aftershock, the vast
majority of aftershocks are located close to the mainshock. As a rule
of thumb, a magnitude 6 mainshock may have aftershocks up to 10 to 20
miles away, while a magnitude 7 mainshock may have aftershocks as far
as 30 to 50 miles away.